1. Field of the Invention
The present invention relates to the technical field of wireless communication and, more particularly, to a wireless receiver system and method with automatic gain control.
2. Description of Related Art
When the radio frequency (RF) amplifier of a receiver operates in the non-linear range, the interferences such as harmonics and inter-modulation are present due to the non-linearity. FIG. 1 is a schematic diagram of a typical RF amplifier operated in the non-linear range. As shown in FIG. 1, when the RF amplifier 110 is operated in the non-linear range, the harmonics are present on the frequency multiples of an input signal. When the present harmonics fall in a range of desired channel, it causes the interference, and thus the quality of received signal in the system is reduced.
FIG. 2 is a schematic diagram of a typical RF receiver. As shown in FIG. 2, an automatic gain control (AGC) system 220 is implemented at the back of a channel filter 210 in order to measure an intermediate signal (IF) strength. Since the channel filter 210 filters out signals except in the desired channel, the desired channel signal mostly contributes to the signal strength measured by the AGC system 220.
When a received signal becomes weaker, the AGC system 220 increases the gain of the RF variable gain amplifier 230 and the gain of the IF variable gain amplifier 240. In this case, when the interference occurs at an adjacent channel, the channel filter 210 filters out the interference, and the desired channel signal mostly contributes to the signal strength measured by the AGC system 220. The input signal to the RF variable gain amplifier 230 includes the desired signal and adjacent channel interference, and in this case the harmonics present on the desired channel, as shown in FIG. 2, when the RF variable gain amplifier 230 is operated in the non-linear range due to the overlarge adjacent channel interference. Thus, the AGC system 220 of the typical receiver limits the gain of the RF variable gain amplifier 230 to thereby avoid the aforementioned problem. However, for a weaker received signal, no sufficient gain can be provided under the limit of the front-end gain, so that the received quality is negatively influenced.
To overcome this, U.S. Pat. No. 6,735,423 granted to Uskali, et al. has disclosed a “Method and apparatus for obtaining optimal performance in a receiver” by measuring the power levels for surrounding channels and a desired channel respectively to accordingly adjust the gain of the AGC device. However, an adjacent channel is necessarily set for measuring the power levels within the surrounding channels, and in this case the signal of the desired channel cannot be received. When the power of the adjacent channel interference is changed, the steps above have to be repeated again; otherwise the quality of the desired channel will be negatively influenced. Therefore, such a way is not suitable for a digital video broadcasting (DVB) system.
U.S. Pat. No. 6,901,243 granted to Jayaraman, et al. has disclosed a “Method and apparatus for mitigating adjacent channel interference in a wireless communication system”, which performs an ADC sampling on a baseband and subsequently uses a digital filter to mitigate adjacent channel interference (ACI). However, such a way cannot improve the interferences such as harmonics and inter-modulation that are caused by the RF amplifier operated in a non-linear range.
U.S. Pat. No. 7,212,798 granted to Adams, et al. has disclosed an “Adaptive AGC in a wireless network receiver”, which compares wideband received signal strength index (wideband RSSI) of an RF signal with the signal strength of a filtered intermediate frequency to thereby detect whether the adjacent channel interference occurs. However, the tuner of a receiver may not be implemented to provide such a wideband RSSI function. In addition, for implementing such a wideband RSSI function, the corresponding wideband RSSI module relatively adds the hardware cost.
Therefore, it is desirable to provide an improved wireless receiver system and method with automatic gain control to mitigate and/or obviate the aforementioned problems.